Earlier this month, Microsoft released MS16-035 that addresses issues I previously reported in SignedXml. They did not only fix the duplicate Id vulnerability I reported though, they also fixed a number of other issues – introducing some breaking changes. This post is an effort to document those and changes and the registry switches that can be used to revert back to the old behaviour.
These are the breaking changes I know about. If you know of any more issues, please leave a comment or drop a mail and I’ll try to update the post.
Duplicate Ids for reference elements no longer allowed (applies to both SignedXml and EncryptedXml)
Id values must be a well formed NCName (which is required by the XML standard, applies to both SignedXml and EncryptedXml)
.NET’s SignedXML class has had a risky implementation for lookup of XML elements by id in GetIdElement() when resolving signed xml references. The lookup validated only the first element if there are several with the same id. This opens up for XML Signature Wrapping attacks in any library that is using the default implementation without taking necessary precautions. For SAML2 libraries signature wrapping is a well known attack class with very severe implications.
I reported this privately to Microsoft on December 3rd 2015. They responded (as promised within 24 hours) that they would investigate. The vulnerability was assigned ids CVE-2016-0132 and MS16-035. A fix was released on “patch Tuesday” in March 2016 (and yes, I’m proud to be listed in the acknowledgement section). The fix also contains a number of related breaking changes.
This is an example of a signed XML document with data that might be incorrectly trusted.
The document demonstrates how two elements have the same id. The unpatched SignedXml.GetIdElement() method will only find and validate the first occurrence of the id, but code that loops all nodes and checks that the id is present in the signature’s references will trust both <b> nodes.
Last week I showed a peculiar XML Signature that validates even though the containing document was changed. The reason is that the signature lacks References. Before explaining what’s wrong with the signature – and with the validation code, we’ll have a look at how XML Signatures work.
XML DSig Primer
XML in general is a powerful beast, with so many options available that it quickly gets really complex. The XML Digital Signatures standard is no exception to that. The extra features complexity of XML DSig compared to other signature standard is that one or more different blocks of data can be signed by the same signature block. That data can be the containing XML Document, part of an XML document or some other resource such as a web page. In this post we’ll only look at signing resources in the document containing the signature.